The invention relates to a resistive current-limiting device having at least one conductor track, which is designed for a predetermined rated current, contains metal oxide high Tc superconductor material, is arranged on a support body and is provided with a cover layer comprising an at least largely insulating material.
In electrical alternating current supply networks, it is impossible to reliably avoid short circuits and electrical spark-overs. In such events, the alternating current in the circuit affected rises very rapidly, i.e. in the first half-wave, to a multiple of its rated value, until it is interrupted by suitable fuse and/or switching device. Consequently, considerable thermal and mechanical loads caused by electrodynamic forces occur in all the affected network components, such as lines and busbars, switches and transformers. Since these brief loads increase proportionally to the square of the current, reliable limiting of the short-circuit current to a lower peak value may considerably reduce the demands imposed on the load-bearing capacity of these network components. In this way, it is possible to achieve cost benefits, for example when constructing new networks and when expanding existing networks, in that it is possible to avoid exchanging network components for designs which can withstand higher loads by incorporating current-limiting devices.
With superconductive current-limiting devices of the resistive type, it is possible, in a manner known per se, to limit the current rise after a short circuit to a value of a low multiple of rated current; furthermore, a limiting device of this type is ready to operate again a short time after switching off. It therefore acts as a rapid, self-restoring fuse. It ensures a high level of operational reliability, since it acts passively, i.e. autonomously, without prior detection of the short circuit and without active triggering by a switching signal.
Resistive super conductive current-limiting devices of the type described in the introduction form a superconductive switching section which is to be connected in series into a circuit. The transition of at least one superconductive conductor track from the virtually resistance-free cold operating state at below the critical temperature Tc of the superconductor material to the normally conductive state above Tc (known as the quench) is utilized, the electrical resistance Rn of the conductor track which is now present limiting the current to an acceptable level of I=U/Rn. The heating to above the critical temperature Tc takes place by means of Joule heat in the superconductor of the conductor track itself if, after a short circuit, the current density j rises to above the critical value jc of the superconductor material, with the material already having adopted a finite electrical resistance even below the critical temperature Tc. In the limiting state at above the critical temperature Tc, a residual current which has advantageously been reduced flows in the circuit until the circuit has been fully interrupted, for example by means of an additional mechanical disconnect switch.
Superconductive current-limiting devices with known metal oxide high Tc superconductor materials (HTS materials for short), of which the critical temperature Tc is so high that they can be kept in the superconductive operating state using liquid nitrogen (LN2) at 77 K, present a rapid increase in the electrical resistance when the critical current density jc is exceeded. The heating to the normally conductive state and therefore the current limiting take place within a sufficiently short time for it to be possible to limit the peak value of a short-circuit current to a fraction of the unlimited current, for example 3 to 10 times the rated current. The superconductive current path should be in thermally conductive contact with a coolant which is able to restore it to the superconductive operating state within a relatively short time of the critical current density jc being exceeded.
Corresponding demands can be largely fulfilled using the current-limiting device which is described in DE-19520205 A1. The known current-limiting device has a support body comprising an electrical insulating material, such as for example Y-stabilized ZrO2 or glass, to which a metal oxide HTS material in the form of a layer structured with at least one conductor track is applied directly or via an intermediate layer. The conductor track may be designed in particular in meandering form (cf. EP 0 523 374 A1). At its ends, the conductor track can make contact with further conductors for feeding in or tapping off the current which is to be limited. Furthermore, in the known current-limiting device, to protect its HTS material against environmental influences such as moisture, it is possible for at least the superconductor material also to be covered with an insulating layer.
Embodiments of current-limiting devices using HTS material in which the conductor tracks are covered with normally conductive material, which serve as shunt resistors, are known (cf. EP 0 345 767 A1).
With current-limiting devices of this type, one technical problem is the dissipation of the thermal energy which is locally deposited in the superconductor and/or metal layers during a switching process: the support body which bears the conductor track in this case serves as the principal heat accumulator during the switching phase, while the heat transfer from the material of the conductor track to a reservoir of the coolant, such as in particular LN2, is low and, moreover, deteriorates further as a result of a film of gas being formed at the surface. It has also been found that, in the first switching phase, prior to the significant onset of thermal diffusion, temperature gradients of more than 100 K/mm are formed between conductor track regions which are still in the superconductive state and regions which have already switched and therefore are being heated from the coolant temperature to a higher temperature level. In this case, the temperature gradients which the layer system of the conductor tracks can locally tolerate ultimately constitute the material-specific limit for a maximum electrical power which is to be switched.
Furthermore, liquid nitrogen (LN2) or the nitrogen gas film which forms as a result of the heating additionally has a dielectric strength which is significantly lower than that of a solid and is of particular significance when used with an increasing switching capacity while at the same time optimizing the utilization of area by minimizing the spaces between individual conductor track parts, for example in a meandering form.
In view of the cooling technology problems outlined above, it has hitherto been necessary to restrict the switching capacity of current-limiting devices of this type to a relatively low value.
Therefore, it is an object of the present invention to design the current-limiting device with the features described in the field of the invention introduction, in such a way that it can be used for relatively high switching capacities.
According to the invention, this and other objects are achieved by the fact that the material of the cover layer is a plastic having at least one filler which increases the thermal conductivity, and at least that part of the cover layer which is associated with the surface of the at least one conductor track has a thickness which is greater than the thickness of the conductor track.
The advantages associated with this design of the current-limiting device are in particular that the extent of temperature gradients in the conductor track is reduced, thereby affecting spatial homogenization of the phase transition. Furthermore, the cover layer which is situated on the front side and if appropriate also on the back side of the structure of the current-limiting device overall, imparts greater mechanical stability to the structure. Furthermore, by suitably selecting the plastic and filler materials, it is possible to ensure a sufficiently high dielectric strength.
This is because a cover layer of this type, which acts as an insulating solid body, during the switching operation functions as an additional heat buffer for the thermal energy which has been deposited in the conductor track. Moreover, compared to a liquid and, in particular, gaseous, turbulently flowing coolant, such as LN2, filled plastic materials have significantly better coefficients of thermal conductivity, heat storage and heat transfer. Moreover, they have the high mechanical stability which has already been mentioned. On account of the improved dissipation of heat out of the conductor track into the applied buffer comprising the cover layer material, local regions of the conductor tracks are heated to a lesser extent during the first millisecond of the switching operation; i.e. the temperature gradients are reduced accordingly. Therefore, the local resistance is lower and the greater current rise is utilized to switch track regions with a higher jc at an earlier time and with lower nominal voltages applying into the resistive state. As a result, the resistance required for current limiting is generated to an increased extent by the increase in switching area the through material. Therefore, the thermal and mechanical loads on the layer system are reduced.
Insulating materials which cure at room temperature or at an elevated temperature and are provided with the filler are advantageously selected as the cover-layer material. In particular, these insulating materials are preferably synthetic resins based on epoxy resin. Materials of this type can be applied relatively easily and without pores to the surface of the conductor track or the structure comprising conductor track and support body, and can be cured there.
The proportion of filler material in the plastic material is advantageously selected to be between 5 and 60% by volume, if electrically conductive filler material is provided. If electrically nonconductive filler material is being used, the filler may form up to 80% by volume. In this way, it is possible to ensure not only a sufficient mechanical stability of the structure comprising cover layer and conductor track below it, but also particularly good dissipation of heat.
The filler materials provided are advantageously at least one material selected from the group consisting of Cu, Ag, Al, their alloys, metal oxides, in particular Al2O3 or Y2O3 or CuO. With these materials, it is possible to achieve a particularly good dissipation of heat to the cryogenic coolant. With a view to achieving a sufficient dielectric strength, filler materials comprising electrically nonconductive material are particularly suitable.
In general, the mean thickness of the cover layer should be between 10 xcexcm and 1 mm. This firstly allows sufficient cooling of the superconductive conductor track; secondly, sufficient account is also taken of the mechanical stability.
The selection of material for the cover layer and in particular for the fillers is advantageously selected in such a way that a dielectric strength of the cover layer at the operating temperature of the superconductor material of at least 15 kV/mm, preferably at least 20 kV/mm, is observed. Dielectric strength of this level can readily be achieved using conventional filler materials and plastic materials.
Further advantageous configurations of the current-limiting device according to the invention are given in the remaining dependent claims.